Recent advances of wireless communications have led to the emergence of new multi-user communication techniques, including multi-user diversity and Successive Interference Cancellation (SIC). SIC is the optimal multiple access scheme to achieve the uplink capacity, see, e.g., D. Tse and P. Viswanath, “Fundamentals of Wireless Communication”, Cambridge University Press, 2005. In a conventional single-decoder receiver, the interference from data streams associated with other users in the network is treated as noise. Differently, in an uplink receiver employing SIC, a data stream associated to a first user is decoded and the corresponding reconstructed data stream is removed from the aggregate received signal, before the next data stream is decoded. This process is repeated for each data stream until all of the data streams in the signal have been decoded. SIC techniques can also be used on the downlink, where data streams intended for different users are simultaneously transmitted and potentially interfere with each other. Using a SIC receiver at a particular user's device, the data streams with the highest signal quality (corresponding to the lowest probability of decoding error) are decoded first and the corresponding reconstructed signals are then successively removed from the received signal, before the data stream intended for the particular user is decoded. SIC processing is described for example in M. K. Varanasi and T. Guess, “Optimum Decision Feedback Multiuser Equalization with Successive Decoding Achieves the Total Capacity of the Gaussian Multiple-Access Channel”, in Proceedings of Thirty-First Asilomar Conference on Signals, Systems, and Computers, vol. 1, pp. 1405-1409, November 1997, and in D. Tse and P. Viswanath, “Fundamentals of Wireless Communication”, Cambridge University Press, 2005. The implementation of a SIC receiver requires a significant use of processing resources. In the downlink, the use of SIC techniques at the user equipment (UE) receiver is therefore limited by its complexity, which scales with the number of users.
In a Multiple-Input Multiple-Output (MIMO) system, spatial multiplexing allows the transmission of multiple data streams (or data layers) over different spatial channels. As is known in the art, multiple transmit antennas can send different data streams over separate spatial channels, and the use of multiple receive antennas can allow the recovery of the different data streams, see, e.g., G. J. Foschini and M. J. Gans, “On Limits of Wireless Communications in a Fading Environment when Using Multiple Antennas”, Wireless Personal Communications, vol. 6, no. 3, pp. 311-335, March 1998, I. E. Telatar, “Capacity of Multi-Antenna Gaussian Channel”, European Transactions of Telecommunications, vol. 10, no. 6, pp. 585-595, November/December 1999, and D. J. Love and R. W. Heath, Jr., “Limited Feedback Unitary Precoding for Spatial Multiplexing Systems”, IEEE Transactions on Information Theory, vol. 51, no. 8, August 2005. As shown schematically in FIG. 1, each transmit antenna 141 and 142 transmits to each (both) receive antennas 161 and 162 at the receiver. Any number of transmit antennas and receive antennas may be used, and the maximum number of data streams that can be distinguished due to the spatial multiplexing is equal to the lower of the number of transmit antennas and the number of receive antennas.
The transmission system shown in FIG. 1 can be described by the equationr=Hx+n  (1)where r denotes the received signal vector, x is the transmitted signal vector, H indicates the MIMO channel matrix, and n is the noise (noise-plus-interference) vector. The channel matrix H models the characteristics of the MIMO propagation channel. In the case of a frequency non-selective channel, Equation (1) can be expanded as
                                          [                                                                                r                    1                                                                                                                    r                    2                                                                        ]                    =                                                    [                                                                                                    h                        11                                                                                                            h                        12                                                                                                                                                h                        21                                                                                                            h                        22                                                                                            ]                            ⁡                              [                                                                                                    x                        1                                                                                                                                                x                        2                                                                                            ]                                      +                          [                                                                                          n                      1                                                                                                                                  n                      2                                                                                  ]                                      ,                            (        2        )            where r1 and r2 are the signals received at the respective receive antennas 161 and 162; x1 and x2 are the signals transmitted from the respective transmit antennas 141 and 142; h11, h12, h21 and h22 are the coefficients of the (frequency non-selective) MIMO channel; and n1 and n2 are the noise (noise-plus-interference) at the respective receive antennas 161 and 162. The noise-plus-interference term typically includes noise generated inside the receiver, which is conventionally modelled by an equivalent stochastic process at the antenna connector.
A successive interference cancellation (SIC) technique can be used to improve the decoding process in the MIMO system. Once a data stream is correctly decoded, the corresponding received signal component can be reconstructed and subtracted from the received signal. Assuming that each data stream is decoded perfectly and the corresponding reconstructed signal is subtracted at each stage of the SIC procedure, then decoding the final data stream is performed with no interference caused by the other data streams in the received signal.
The SIC technique is applicable to both cases of Multi-User MIMO (MU-MIMO) transmission, where the multiple spatially separated data streams are simultaneously transmitted to multiple users, and Single-User MIMO (SU-MIMO) transmission, where the multiple parallel data streams are transmitted to a single user.
The SIC technique is particularly sensitive to error propagation. If an error occurs in decoding or subtracting one of the data streams, then the subsequent data streams to be processed are likely to be affected by an increased probability of decoding errors. The ordering of the data streams can affect the error propagation. In a MIMO system where all of the data streams are equally protected by channel coding, the data streams are decoded starting from the strongest data stream (in a Signal to Noise Ratio sense or based on other suitable metrics) and continuing by decoding progressively weaker data streams in sequence.